This invention relates generally to semiconductor devices and manufacturing methods and more particularly to semiconductor devices formed by vapor phased epitaxy.
As is known in the art, semiconductor devices such as field effect transistors are often employed to amplify radio frequency power by feeding an r.f. voltage signal to a gate electrode to control the conductivity of a drain-source channel underlying the gate electrode. As such, radio frequency performance is dependent upon the quality of the crystalline structure of the semiconductor layers used to form the field effect transistor. As is also known in the art, Group III-V semiconductor materials system such as a system employing gallium arsenide (GaAs) are often used to fabricate field effect transistors for amplifying radio frequency power.
One technique used in the prior art to provide high quality field effect transistors is to grow active regions for the field effect transistors directly on GaAs substrates. A substrate comprising gallium arsenide for use in fabricating such field effect transistors generally is prepared with a relatively high bulk resistivity typically in the range of 10.sup.7 -10.sup.8 ohm-cm. This relatively high resistivity is required so that field effect transistors will be fabricated with relatively low leakage currents. Leakage current affects field effect transistor r.f. performance since a leakage current cannot readily be controlled or modulated by the r.f. voltage signal applied to the gate electrode.
Generally, two methods are employed in the prior art to prepare substrates of GaAs having a relatively high bulk resistivity. One method, the so-called Horizontal Bridgeman Technique involves the steps of introducing elemental Ga and As into an open quartz reaction vessel, reacting the Ga and As to form a GaAs melt, and slowly withdrawing the sealed vessel from the furnace to form a crystalline structure. The crystalline structure is sliced into wafers which are then lapped and polished. Because residual donor ions originating from the quartz reaction vessel are now present in the crystal reducing the crystal's resistivity, this method generally requires doping with a material such as chromium to provide relatively high resistivity substrates having relatively low dislocation densities. Chromium, having electrons at an energy level intermediate the crystal's valance band energy level and the crystal's conduction band energy level, is a deep level acceptor which neutralizes the residual donor ions to thereby increase the crystal's resistivity. One problem associated with chromium doping is that the rate of recombination of electrons and holes between the valance band and intermediate energy level of the chromium is lower than the rate of change of an injection current flux in the conduction band resulting in a net fixed negative charge of chromium ions. At the active layer/substrate interface, these chromium ions repel electrons in the channel of the device resulting in a loss of power. A second problem associated with chromium doping is that chromium has a tendency to out diffuse from the substrate into the active layer/substrate interface region resulting in a decrease in electron mobility and degraded device performance.
In a second method, the so-called "Czochralski Technique," a seed crystal is slowly withdrawn from a GaAs melt in a controlled atmosphere. This technique is particularly useful in providing relatively large circular substrates of GaAs. Several variations of this technique have been developed. The more widely used one is the so-called "Liquid Encapsulated Czochralski Technique" where the seed crystal is pulled through a layer of melted boron oxide which acts an an encapsulant to assist in the prevention of arsenic from leaving the melt, a problem generally common to all variations of the "Czochralski Technique." As described in an article entitled "Compensation Mechanism in Liquid Encapsulated Czochralski GaAs: Importance of Melt Stoichiometry" published in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 7, July 1982, by D. E. Holmes, R. T. Chen, K. R. Elliott, C. G. Kirkpatrick, and Phil Won Yu, carbon atoms provided by chemical contaminants which are present in the GaAs melt used to form the GaAs substrate act as electrically active impurities which compensate for a defect in the stoichiometry of the substrate. The stoichiometric defect often denoted as "EL2," is a result of extra or interstitial arsenic atoms being present in the crystal lattice. Each one of such arsenic atoms has an electron at an energy level which is intermediate the valance band energy level and the conduction band energy level of the GaAs crystal. When ionized by an electron current flux, the extra or interstitial arsenic atoms provide such electrons into the conduction band of the crystal thereby reducing the resistivity of the substrate and providing positive ions in the crystal. The presence of carbon compensates for this electron by accepting an electron from the valance band of the crystal thereby creating a hole current in the valance band and providing a negative ion in the crystal. As described in the above-mentioned article, the high resistivity of the substrate is provided by controlling the ratio of arsenic to gallium in the melt to thereby provide a controlled concentration of stoichiometric defects.
However, the quality of the crystalline structure of substrates fabricated by either method, particularly by the Czochralski technique are generally not suitable for fabrication of high quality field effect transistors directly thereon, because inside the crystalline structure close to the surface of the substrate unwanted crystalline defects such as hole-and-electron traps are present which can degrade the electrical properties of a device fabricated directly thereon. These traps can become ionized sites when they accept or emit an electron. Thus, during operation of a field effect transistor, the electric field created by ionization of traps will restrict the flow of electrons in the channel, an effect generally known in the art as "backgating," with a concomitant loss in power. Currently, therefore much work is going into techniques which will provide high quality gallium arsenide substrates having relatively high resistivities and relatively high crystal quality.
As is also known in the art, a second alternate solution to the problem of fabricating low leakage field effect transistors which overcomes the low crystalline quality for GaAs substrates is to first grow a high resistivity buffer layer over the substrate surface and then grow the active regions of the field effect transistor on the buffer layer. The buffer layer provides a high quality, high resistivity layer which shields the active regions of the field effect transistor from defects in the GaAs substrate crystal. The buffer layer preferably should have a high resistivity and also should be relatively thick to adequately isolate the active regions from crystal defects present in the crystalline structure of the substrate.
One method suggested for accomplishing this is the use of a vapor-phase epitaxial method wherein a liquid gallium source is saturated with arsenic gas to form a crust of highly pure gallium arsenide. One problem associated with this method is that the saturation condition for the liquid gallium source as well as the topographic structure of the crust is very difficult to control due to the effect of convection in the liquid gallium and the arsenic containing gas. Consequently, the ratio of gallium to arsenic is also difficult to control, giving rise to changes in the stoichiometry of the epitaxially grown crystal layer. These stoichiometry changes result in the above-mentioned stoichiometric defect, that is, the extra or interstitial arsenic atoms. It is believed that the resistivity of such substrates is provided by compensating for such stoichiometric defects with carbon which is provided on the GaAs substrate from contaminants in a similar manner as described in the above-mentioned article. The carbon on the surface of the substrate out diffuses as the thickness of the epitaxially grown buffer layer increases so that the concentration of carbon correspondingly decreases as the thickness of the buffer layer increases. Because the active layer is formed epitaxially on this intermediate high resistivity buffer layer, and because of the difficult nature of controlling the concentration of carbon impurities on the surface of the substrate, epitaxial growth of buffer layers greater than 1-2 .mu.m having predetermined resistivities is generally difficult to obtain. Thus, the resistivity of buffer layers obtained by using a liquid gallium source method is typically difficult to control and also difficult to reproduce being dependent upon the amount of contaminants present in the substrate as well as the growth conditions for the epitaxial layer. Therefore, because of the thin buffer layers, the active regions of the field effect transistors grown on such buffer layer are close to the substrate/buffer layer interface, and thus the buffer layer will not generally adequately isolate the active region from the defects in the substrate crystal. Thus, during operation of a field effect transistor, electrons in the channel are not adequately shielded from the electric field created by traps present at the substrate-buffer layer interface thereby constricting the channel of the field effect transistor and resulting in a loss of power.
A second method suggested in the art is to grow a doped buffer layer using chromium as a dopant. For the reasons discussed above in reference to substrate doping, chromium or a similar "deep level" acceptor is not an adequate solution in certain applications because the rate of recombination of electrons and holes between the valance band and the acceptor atom is often lower than a rate of charge of an injected electron flux in the conduction band. This results in a fixed negative charge at the buffer layer/active layer interface which repels electrons in the active layer thereby constricting the channel. Further, it is also believed that chromium has a tendency to out diffuse from the buffer layer into the active region. This diffusion is undesirable because it results in a decrease in electron mobility which affects current density and thus device performance.